Membrane protein biosynthesis in the retina is being analyzed by a new immunochemical approach. We have previously shown that two intrinsic membrane proteins of retinal rod outer seqments (ROS) are synthesized and transported on membranes from the Golgi apparatus to the ROS. Besides opsin, the visual pigment, a new large protein has been isolated and localized to the incisures of ROS discs. Accordingly it has been named "incisin". Antibodies to opsin and incisin have been prepared. These will be used a probes for subcellular fractionation of photoreceptor cell membranes. By attachment of these antibodies to solid substrates we hope to separate membranes bearing these proteins from other subcellular membranes by affinity column techniques. As an alternative, we will couple polyethylene oxide (PEO) directly to the antibodies or indirectly by an avidin-biotin complex to influence the partitioning of the antibodies in liquid two-phase gradients of PEO and Dextran. If PEO-antibodies are partitioned to the PEO (upper) phase they may serve as suitable immunoaffinity partitioning reagents for isolation of membranes bearing newly synthesized opsin and incisin. Antibodies will also be prepared to a portion of opsin's amino terminal sequence. These sequence specific antibodies can be expected to react with this region of opsin on either the interdiscal or intradiscal face of ROS discs. By this means, we may determine opsin's orientation in the disc. Then by reaction with newly synthesized opsin, we will be able to probe its orientation during transport, prior to assembly in ROS membranes.